Field of the Invention
The present invention relates to a printed wiring board including a transmission line and also relates to a printed circuit board including the printed wiring board.
Description of the Related Art
In a printed wiring board, terminals of a plurality of mounted semiconductor integrated circuits are connected to each other by a transmission line to carry out a signal transmission. In the signal transmission, a waveform of the signal transmitted from the semiconductor integrated circuit is preferably transmitted to the receiving-side semiconductor integrated circuit without being distorted while the signal is transmitted on the transmission line. For that reason, a characteristic impedance of the transmission line is preferably set to be uniform as much as possible to avoid a reflection corresponding to a cause of the waveform distortion.
Meanwhile, because of demands for a miniaturization of electronic equipment in recent years, a multilayer printed wiring board including a plurality of conductive layers on which a signal wiring pattern used for the signal transmission is arranged is being used for the printed wiring board. In a case where the signal transmission between the semiconductor integrated circuits mounted on different layers on front and back sides is carried out in the multilayer printed wiring board, wiring is to be laid out by switching the conductive layers on which the signal wiring pattern is arranged by a via conductor.
Since such via conductor is a discontinuous structure with respect to the signal wiring pattern, the setting of the uniform characteristic impedance is obstructed. Up to now, an operating frequency is low, and a time during which the signal passes through the via is short with respect to a rising time of the signal, so that an influence thereof can be ignored.
However, along with realization of a higher speed of a circuit operation in recent years, nonuniformity of the characteristic impedance caused by this via conductor becomes difficult to ignore.
As a method of solving this, a method of setting uniform characteristic impedance by connecting front and back signal wiring patterns to each other by two adjacent via conductors is proposed (see Japanese Patent Laid-Open No. 2007-123742). Whereas the characteristic impedance of the signal wiring pattern is approximately 40[Ω] to 60[Ω], the via conductor is approximately 120[Ω] in terms of the characteristic impedance which is higher than the signal wiring pattern, and these characteristic impedances are not uniform.
According to Japanese Patent Laid-Open No. 2007-123742, the connection is realized by the via conductors where the characteristic impedance is effectively approximately halved by arranging the via conductors at the characteristic impedance of 120[Ω] in parallel. For this reason, it is possible to set the characteristic impedances of the two via conductors to be close to the characteristic impedance of the signal wiring pattern.
The characteristic impedance of the transmission line preferably has a low value of a difference (variation width) between a maximum value and a minimum value. For example, the variation width of the characteristic impedance of the transmission line is preferably suppressed to be 20[%] or lower (or more preferably, 10[%] or lower).
However, according to Japanese Patent Laid-Open No. 2007-123742, the characteristic impedance at the part corresponding to the two via conductors is set to be close to the characteristic impedance of the signal wiring pattern, but the variation width of the characteristic impedance of the transmission line still exists, and this variation width is preferably decreased further.
In view of the above, the present invention provides a printed wiring board and a printed circuit board with which it is possible to further set the characteristic impedance of the transmission line to be uniform.